a = int(input())
max_a = 0
for start in range(4):
      for end in range(start, 4):
            str_a = str(a)
            n = int(str_a[start:end + 1] )
            flag = 1
            for d in range (2, int(n ** 0.5)  + 1):
                  if n % d == 0:
                       flag = 0
            if flag == 1 and n > max_a:
                max_a = n
if max_a != 0:
     print (max_a)
else:
     print (0)
